In this article, laser phase transformation hardening of HT250 material by high speed axis flow CO2 laser is investigated. Appropriate laser hardening parameters, such as laser power P (W),laser scanning rate V (m/min), have been optimized through a number of experiments. The effects of the parameters on the hardened zone, including case depth, micro hardness distributions etc. are analyzed. The experiment shows that the axis flow CO 2 laser, which commonly outputs low mode laser beam, also can heat treat materials as long as the treating parameters used are rational. During the experiments, the surface qualities of the specimens treated with optimized parameters were enhanced, which incoincided with former results. According to the experimental results, the relation between the optimized parameters, the laser power densities q and the scanning rates V is given.